The present invention relates to X-ray systems utilizing a solid state multiple element X-ray detector for producing an image; and more particularly, to techniques for reading the signals from the detector elements and processing the signals.
Conventional fluoroscopy equipment includes a source for projecting an X-ray beam through an object to be analyzed, such as a medical patient. After the beam passes through the patient, an image intensifier typically converted the X-ray radiation into a visible light image, and a video camera produced an analog video signal from the visible image for display on a monitor. Because an analog video signal was produced, much of the image processing for automatic brightness control and image enhancement was performed in the analog domain.
Recently, high resolution solid state X-ray detectors have been proposed, which comprise a two dimensional array of 3,000 to 4,000 photo diode detector elements in each dimension. Each element produces an electrical signal which corresponds to the brightness of a picture element in the X-ray image projected onto the detector. The signal from each detector element is read out individually and digitized for further image processing, storage and display.
The high resolution provided by such a large array of detector elements may be beneficial to the analysis of the patient in many certain circumstances. However, in situations where a high degree of resolution is not required, the processing of the incumbent large amount of data from the array significantly increases the image processing time above that which is reasonably required. Furthermore, certain X-ray systems may not require a detector with such a high degree of resolution in which case a smaller matrix detector would be satisfactory. However, in this instance it may be cost effective to still utilize the larger matrix detector, rather than develop a different size detector for each piece of X-ray equipment having a different resolution requirement. In both these cases when the detector produces a higher resolution image than is required, a technique for reducing the amount of data is desirable.
A problem inherent in conventional photo diode detector arrays is that the biasing charge placed on the photo diode may be partially discharged by transistor current leakage and by a mechanism commonly known as "dark current". The charge depletion due to the effects of dark current and current leakage produces an offset of the image signal. Since the amount of charge removed by these currents is not constant, the signal offset varies, adding a element of uncertainty to the detector output.
The amount of charge removed from the photo diode by these currents in part is a function of the length of time from photo diode biasing until detector element charge sensing. As a consequence, it is desirable to minimize the time required to read out the elements of the detector array in order to minimize the effect of these currents. This presents a trade-off in that it is also desirable to increase the readout time in order to reduce the bandwidth of the image signal processing circuit and reduce the electrical noise added to the detector signal by the circuit.